The present invention relates to level measuring probes for measuring the level of a liquid contained in a reservoir, for example a motor vehicle reservoir.
More precisely, the invention relates to probes having a detector element constituted by a resistive conductor having a high temperature coefficient intended to be immersed in the liquid, and connection means connecting this element to a relatively long connector.
The measurement principle of such probes is simple: a resistive metallic element having a high temperature coefficient has its resistance substantially increased as a function of its temperature. On the other hand, when such a resistive element is immersed in a liquid, its resistance depends on the level of the liquid, since the part of the resistive element disposed above the liquid, and thus exposed to the air, is much less cold than the immersed part. In these conditions, the placing of such an element in an electric circuit supplying the detector provides a level measuring means.
By way of example, a particular circuit is described in French patent application No. 76 30 120 of Oct. 6, 1976, published under the French Pat. No. 2,367,276 in the name of Jaeger.
Two constructions of probe are presently available on the market.
One construction presents, as a detector element, a wire of resistive material having a high temperature coefficient wound in a spiral and as a conducting element two copper connecting wires, these wires being respectively protected by a cap and a semi-rigid sleeve connected the one to the other by a screw system.
The other construction, like that described in French patent application No. 82 04050 published under French Pat. No. 2,501,860, in the name of Jaeger, has a detecting strip disposed on a substrate, forming a printed circuit and connected by connection wires to a processing circuit. Such a construction, as previously described requires means for protecting the printed circuit as well as connection wires.
The cost price of these probes is relatively high because the method of manufacture dictated by the components involves individual manual assembly.
Further, the components used occupy a negligible space which on account of the small space available in use, represents a certain inconvenience.